The present invention relates to both process and apparatus for the recovery of ammonia during the separation of nitrogen oxides NO.sub.x from the fuel or waste gases obtained during the combustion of flammable materials, particularly fossil fuels, in a firing plant wherein a reducing gas, preferably ammonia NH.sub.3 is introduced into the waste gas flow in a reaction zone and through which the nitrogen oxide gases are reduced to elementary nitrogen N.sub.2, and wherein the waste gas flow undergoes wet waste gas cleaning performed after the reaction phase and the washing water removes the unreacted ammonia, called slip, from the waste gas.
Waste or flue gases contain to a greater or lesser extent oxides of nitrogen NO.sub.x, particularly nitrogen monoxide NO in amounts exceeding 90%, which is formed in air when fuels of any type, but in particular fossil fuels are burned. If the flue gases, before undergoing cleaning, are given off to the atmosphere nitrogen dioxide NO.sub.2 is formed from the nitrogen monoxides under the influence of oxygen from the air an/or from the flue gases. When the nitrogen dioxide, which is readily water-soluble gas, is exposed to moisture such as rain, the gas is washed out as nitrous acid HNO.sub.2 or nitric acid HNO.sub.3 and consequently not inconsiderably contributes to the formation of acid rain. The ozone content of the atmosphere is also increased by a photochemical reaction of nitrogen oxides NO.sub.x.
Various process are known for separating nitrogen oxides from waste gases. The most frequently used processes employ ammonia for this purpose which is introduced into the waste gas flow, reacting there with the nitrogen oxides and reducing them to elementary nitrogen N.sub.2.
In one known process, called the SNCR process (Selective Noncatalytic Reduction Process), cf. EP 79 171, ammonia gas is introduced into the furnace chamber of a firing plant. A temperature of 700.degree. to 1100.degree. C. is required to permit the reduction of the nitrogen oxides.
In another known process, called the SCR process (Selective Catalytic Reduction Process), as disclosed for example in U.S. Pat. No. 3,970,739, the ammonia is admixed with the waste gas in a reaction chamber provided with suitable catalysts, temperature of 150.degree. to 700.degree. C. being maintained in the reaction chamber.
It is known that, in both these processes, some ammonia will not react with the nitrogen monoxide. The higher the desired degree of separation, the larger the unreacted amount of ammonia. The unreacted amount can exceed the amounts which are permitted by existing regulations to pass into the atmosphere. Thus, the nitrogen oxide separation efficiency of the known processes is limited by the amount of unreacted ammonia which can be discharged into the atmosphere.